Commands can take other variables too.
http://192.168.1.38:11000/Play?id=0 (play 1st song from playlist)
http://192.168.1.38:11000/Play?seek=60 (jump to 60th second in the playing track)

http://192.168.1.38:11000/Volume?level=0 (value is % so 0 is mute or -91db and 100 is full volume or 0db)
http://192.168.1.38:11000/Repeat?state=0 (different repeat states 0,1,2)
http://192.168.1.38:11000/Shuffle?state=0 (different repeat states 0,1)

The controllers seem to issue the following commands every second alternatively to get their status.
http://192.168.1.38:11000/SyncStatus
http://192.168.1.38:11000/Status

And then using the returned values of those commands they do other stuff like get the Artwork for the song playing.
http://192.168.1.38:11000/Artwork?service=LocalMusic&album=&artist= (insert album and artist name in the URL)

Grouping and Ungrouping of players are done with the
http://192.168.1.38:11000/AddSlave?slave=192.168.1.41&group=Study%20Player (Study Player is the group name)
http://192.168.1.38:11000/RemoveSlave?slave=192.168.1.41

You can get the list of Services that the player can use using
http://192.168.1.38:11000/Services
Or browse through the Radio Service using
http://192.168.1.38:11000/RadioBrowse

This command appears to provide a list of all the Inputs
http://192.168.1.38:11000/RadioBrowse?service=Capture

Then you can switch to them using
http://192.168.1.38:11000/Play?url=Capture%3Ahw%3A1%2C0%2F1%2F25%2F2&preset_id&image=/images/inputIcon.png (Optical Input)
http://192.168.1.38:11000/Play?url=Capture%3Abluez%3Abluetooth&preset_id&image=/images/BluetoothIcon.png (Bluetooth Input)